This study is an open-label, randomized, cross-over study to compare the pharmacokinetics of sulfadiazine given on a twice daily or four times daily schedule. Ten human immunodeficiency virus-infected patients with CD4 counts below 500 will be enrolled. Patients must not have concurrent opportunistic infections and must be willing to comply with the study regimen. The study is designed to compare pharmacokinetic parameters and serum concentrations of sulfadiazine at two doses: 1000mg QID and 2000mg BID. Additionally, this study will assess the safety and tolerability of these dosage regimens. Currently, the study has received IRB approval and will begin enrolling within the next 1 to 2 months.